Nonwoven fabrics suitable for filtration of liquids are well known in the prior art. Typically, filtration pads, socks, or tubes are formed from a surface bonded or binder-impregnated sheet of nonwoven fibers. The filter sheet is fabricated to have a porosity for filtering sediment and other contaminant particles from a liquid, for example, as is done for raw milk from a dairy farm. It is generally desired that such filtration fabrics have good wet strength over prolonged use, have consistent pore size, provide effective filtration at a normal flow speed (low pressure drop) through the filter, and be economical to manufacture.
Nonwoven filtration fabrics have been made of cellulosic fibers, but such fibers swell in water and tend to weaken or collapse in use. Crimped rayon fibers have been added to strengthen the fabric, but the resiliency of the fibers under wet process conditions has not been satisfactory. In order to improve the characteristics of the filter fabric, various blends of fibers have been used.
For example, in U.S. Pat. No. 2,971,907 to Smith, glass monofilament fibers are blended with fine inorganic fibers, such as asbestos or glass fibers, and impregnated with a binder. In U.S. Pat. No. 3,307,706 to Taylor, cellulosic fibers of low denier are blended with polymeric fibers of a high wet stiffness, and impregnated with a binder. It is known that of the various nonwoven sheet forming processes, a wet forming process is the most beneficial for industrial production of nonwoven sheets made of polymeric fibers. However, the resultant filter fabrics are still considered not fully satisfactory in mechanical strength, surface quality, uniformity of fiber, filtration effectiveness, and/or durabiltiy.
A recent development, as disclosed in U.S. Pat. No. 4,496,583 of Yamamoto et al., is a polyester fiber sheet made by a wet sheet-forming process from drawn, uncrimped polyester staple fibers of low denier (0.9 or less) and short length (15 mm. or less), alone or combined with undrawn polyester of low denier and short length and a small amount of other staple fibers. The polyester fibers of low denier are used to avoid frequent breakage during the wet forming process, undesirable fluffs and stiffness, and poor mechanical strength. The short length is used to avoid poor dispersal of the staple fibers in water when they become entangled with each other. However, filter sheets formed of the low denier, short length polyester fibers have the disadvantages of small pore size and high pressure drop through the filter, making them unsuitable for filtering larger particles (over 2 microns) at normal flow rates, such as for filtering milk.
Accordingly, it is a principal object of the invention to provide an improved nonwoven filtration fabric having the desired properties of high strength, uniformly distributed pores, low pressure drop, good filtering properties, and suitability for macrofiltration.
A further object of the invention is to manufacture a nonwoven filtration fabric with high productivity, consistency and quality using a wet sheet forming process.